Ruthenium is a noble metal (like platinum or iridium) and has been extensively studied for use as a conductive layer of a highly integrated memory device. In particular, a ruthenium conductive layer may be used in the formation of an electrode of an MIM-type capacitor. In a highly integrated memory device such as a DRAM or FRAM, it is important that each of the capacitors has a large capacitance while occupying a small area. In order to increase the capacitance, a method for increasing a surface area of a capacitor electrode or a method for reducing a thickness of a dielectric layer may be employed. In addition, materials with high dielectric constants may be used so as to increase the capacitance. As examples of materials having high dielectric constants, there are metal oxides such as TaO and TaON, and perovskite materials such as BST, PZT, and the like. The metal oxides such as TaO and TaON have dielectric constants amounting to several times to several hundred times as high as that of a silicon oxide layer.
However, when a capacitor electrode layer and the capacitor dielectric layer are made of polysilicon and a material having a high dielectric constant as discussed above, respectively, a surface of the polysilicon may be oxidized during the process of forming the layers. In addition, in order to obtain a stable leakage current characteristic, a low k-dielectric layer such as a silicon nitride oxide layer must be formed to prevent reaction between the polysilicon and a high k-dielectric layer. However, because the silicon nitride oxide layer has a low dielectric constant, the thickness of the dielectric layer is substantially increased and the capacitance is substantially reduced. On the other hand, when the electrode of the MIM capacitor comprises a metal having a work function that is larger than that of the dielectric layer, a leakage current barrier may be formed due to a difference in the work function between the dielectric layer and the electrode. Therefore, it is unnecessary to additionally form a leakage current barrier layer that may induce a decrease in the capacitance.
The electrode of the MIM capacitor is normally composed of a material selected from the group consisting of noble metals such as platinum, ruthenium, and iridium and their metal oxides. The noble metals and their metal oxides have large work functions and do not react on the dielectric layer. In particular, unlike other noble metals, ruthenium may be readily etched by a plasma etching process using oxygen. In addition, even if oxidized, the resulting ruthenium oxide still has conductivity. In this respect, ruthenium has been recognized as a material more suitable for forming the capacitor electrode than other materials.
In the fabrication of a semiconductor device, a material like bis(cyclopentadienyl)ruthenium [Ru(EtCp)2] or ruthenium acetylacetonate [Ru(OD)3] is known as a source material of the chemical vapor deposition (CVD) process needed for forming a ruthenium layer. The source materials may be represented by structural formulas as shown in FIG. 1. Unfortunately, the ruthenium source materials may be easily oxidized under typical circumstances. In addition, because the cost of ruthenium itself is high and the fabrication of a ligand of the source material is difficult, the cost of the source material is typically high.
Furthermore, if the ruthenium layer is formed using a conventional source material like Ru(EtCp)2, it is difficult to form a thin layer for the capacitor electrode using a one-step process. In other words, as both the deposition pressure and an oxygen flow rate increase, the number of ruthenium nuclei per unit area increases. Thus, the ruthenium layer forms a columnar structure. The ruthenium layer with the columnar structure has a rough surface and suffers a high sheet resistance as well as a high leakage current. By contrast, as both the deposition pressure and the oxygen flow rate decrease, the number of the ruthenium nuclei per unit area is reduced, although the nuclei are isotropically formed. Hence, the electrical resistance of the ruthenium layer is increased.
Therefore, in order to form the ruthenium layer using a conventional ruthenium source material like Ru(EtCp)2, the CVD process should include at least two steps. However, if a two-step process is performed in-situ, the process conditions may become unstable between the first and second steps. This may result in degradation of reproducibility or reliability of the process. In addition, a cooling gas injected into a chamber during the process may change the process pressure, thus permitting the position of a wafer to be varied. As a result, characteristics of the ruthenium layer may be varied. Meanwhile, if the second step is performed in another apparatus, the wafer should be shifted from a first apparatus into a second apparatus and the process conditions must be newly adjusted in the second apparatus. As a result, process efficiency may be lowered.
It is well known to those skilled in the art that if Ru(OD)3 is used as the source material, a one-step process may be performed. At this time, however, because the ratios of carbon and oxygen contained in Ru(OD)3 are high, there may arise problems caused by impurities like carbon. That is, the ruthenium layer may suffer deterioration of both an electrical characteristic and a step coverage characteristic.